Cells stop growing when they reach a certain "saturation" density. The precise mechanisms that control this density-dependent arrest process are not known. However, cell to cell contact is probably important, since the addition of isolated membranes will stop the growth of many kinds of cells. The long range goal of this research is the elucidation of the mechanisms of growth control, knowledge that should be crucial in understanding cancer. The research discussed in this proposal is aimed at studying the molecule in lymphoid cell membranes that inhibits lymphoid cell growth, with experiments focused on 3 major areas. I. The inhibitor is known to be a lipid-like molecule, and high performance liquid chromatography, thin layer chromatography, and mass spectroscopy will be used for purification and structural analysis of the inhibitor. II. The metabolism of inhibited cells will be studied in several ways. Protein synthesis in the presence of inhibitor will be measured using radioactive amino acids, and the synthesis of a single specialized protein, IgG, will be studied using a B-cell hybridoma line. Cytofluorometric analysis, coupled with fluorescent dyes that bind DNA or RNA, will be used to determine the cell cycle position of inhibited cells. Interleukin 2 will be used as a model growth factor: using a cell line that is dependent on this factor, it will be possible to study the interaction of stimulatory and inhibitory signals in regulating cell growth. III. The presence and activity of the inhibitory molecule on different cell types will be investigated. Fibroblasts will be used as a source of material for purification of the inhibitor. Conversely, fibroblasts and other cell types will be tested for growth in the presence of the lipid-like inhibitor. These experiments will examine the possibility that the molecule(s) and recognition systems involved in the control of lymphoid cell growth have similar functions in many cell types. Regardless of whether this growth control system is common to many cells or unique to lymphoid cells, a detailed understanding of the inhibitory molecule and the metabolic processes it effects will be of general interest.